Filters frequently have to meet very difficult standards. For example, filters for use with internal combustion engines such as are conventionally used on heavy duty vehicles should have a very high dirt capacity, especially when operating in a dust-laden atmosphere, and a low pressure drop across the filter so as not to interfere with the efficient working of the engine. At the same time, these filters must fit within a relatively confined space. A conventional filter for use in such applications comprises a cylindrical pleated filter. A dirty fluid, such as dust-laden air or lube oil containing particulate matter, is passed through the cylindrical filter, either from the outside to the inside or vice versa, and from there passes into the engine. The dirt collects on and in the filter medium of the filter. Therefore, the configuration and surface area of the filter medium have a great impact on the dirt capacity of the filter, i.e., the amount of dirt that can be removed before the filter is completely or effectively clogged.
In an attempt to make the dirt capacity of the filter as large as possible, the pleats have been made quite deep and numerous However, it is found that, practically speaking, the depth of the pleats should not exceed about 2 inches to avoid pleat collapse and minimize pressure losses during use. In addition, placing too many pleats around the circumference results in closed-up pleats, especially at the bottom of the pleats, and consequently restricted fluid egress from the filter. This results in significant pressure losses. Therefore, there are practical limitations on steps that can be taken to increase the effective surface area of the conventional pleated filter, without at the same time increasing the diameter of the filter cylinder. This is usually unacceptable since it results in the filter requiring more space than is available. Such constraints have placed severe restrictions on the design of conventional cylindrical filters. Many of the same considerations limit the design of conventional cylindrical fluid filters in other areas such as filters for turbine lubricating oil and for water.
There is, therefore, a need in the art for a cylindrical fluid filter which has an increased filtering surface area within a given amount of space and at the same time an increased dirt capacity. This need is fulfilled by the filter of the present invention.